How much work was done by friction on the person?

In summary, the person lost 1160J of energy when they climbed the 4.8m ladder of a slide and reached a velocity of 3.2 m/s.
  • #1
physicsman2
139
0

Homework Statement


A person's mass is 28 kg. They climb the 4.8m ladder of a slide and reach a velocity of 3.2 m/s at the bottom of the slide. How much work was done by friction on the person?


Homework Equations


KE=1/2mv^2
PE=mgh
W=Fd
i think these are all the equations that apply

The Attempt at a Solution


i think you could find the potential energy=28(9.8)(4.8), which equals the kinetic energy,
then i don't know what to do but i know what i did is wrong
thank you for any help
 
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  • #2


Use the work energy principle,

[tex] \Delta E_k=W [/tex]

What forces are doing work on the person?
 
Last edited:
  • #3


i think you would have to solve for PE first but I am not sure
 
  • #4


Sure, that is a good first step.

How about this, IF there was no friction, how fast would you expect the person to be going at the bottom of the slide?
 
  • #5


she would be going about 10 m/s without friction
 
  • #6


Okay, so then how much kinetic energy would she have if there was no friction? (v=9.7m/s)

How much kinetic energy does she actually end up with?

What happened to the energy?
 
Last edited:
  • #7


she would end up with 1300J of kinetic energy when v = 9.7 m/s

she would have 140 J of kinetic energy with the given 3.2 m/s
 
  • #8


So if you consider that only gravity and friction do work on the person, what can you say.

Remember the conservation of energy.
 
  • #9


you could say that the gravitational potential energy is 140 J since PE=KE
 
  • #10


No, that's not right. The gravitational potential energy at the top is mgh=1300J

The kinetic energy at the bottom is 140J

The difference is 1300J-140J = 1160J

Energy cannot be destroyed, so somehow 1160J of energy was lost by the person.

We haven't yet considered friction though, as the person slides down the slide friction does negative work on the person, slowing them down.
 
  • #11


oh okay, sorry, you're right
it makes more sense now thanks for your help again
 
  • #12


np, don't be sorry! I just wanted you to understand that the only two forces acting on the person are the gravitational force which does work in the amount mgh=1300J, and the frictional force.

Since we know [tex] \Delta E_k=W [/tex], we have [tex]1/2mv_2^2-1/2mv_1^2=1300J - W_f [/tex] where [tex]W_f[/tex] is the work done by friction. It is negative because it acts opposite to the motion of the person :)
 

1. How is work defined in physics?

Work in physics is defined as the product of force and displacement. In other words, work is done when a force is applied to an object and causes it to move a certain distance.

2. Can friction do work on an object?

Yes, friction can do work on an object. When an object is moving and experiences friction, the force of friction acts in the opposite direction of the object's motion, causing it to slow down and eventually stop. This means that friction is doing work to decrease the object's kinetic energy.

3. How is the amount of work done by friction calculated?

The amount of work done by friction can be calculated using the equation W = Fd, where W is work, F is the force of friction, and d is the distance the object moves while experiencing friction.

4. Does the amount of work done by friction depend on the surface area of contact?

Yes, the amount of work done by friction can be affected by the surface area of contact between two objects. Generally, larger surface area means more friction, which results in more work being done.

5. Can friction ever do positive work on an object?

Yes, friction can do positive work on an object in some cases. For example, when a car is driving up a hill, friction between the tires and the road helps the car to move forward, meaning that friction is doing positive work in the direction of the car's motion.

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